The most common way to predict a due date is to count 280 days (40 weeks) from the first day of your last menstrual period. That gives you an estimated due date, but it’s just that: an estimate. Only about 30% of babies arrive on or after their due date, and the prediction itself can shift depending on your cycle length, when you actually ovulated, and what an early ultrasound shows.
The Standard 280-Day Calculation
The formula most calculators and providers use is called Naegele’s rule. Johns Hopkins Medicine breaks it into three steps: take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. If your last period started on March 10, you’d count back to December 10, then add a year and seven days to land on December 17 as your estimated due date.
This formula assumes a 28-day menstrual cycle with ovulation on day 14. If your cycles are consistently longer or shorter than 28 days, the estimate will be off. A 35-day cycle, for example, means you likely ovulated a week later than the formula assumes, pushing your true due date about a week further out. If you know your cycle length, you can make a simple adjustment: instead of adding 7 days in the final step, add the number you get by subtracting 21 from your usual cycle length. For a 35-day cycle, that’s 35 minus 21, so you’d add 14 days instead of 7. This correction, known as Parikh’s formula, was specifically designed to reduce errors for people with irregular or non-standard cycles.
Why Ultrasound Often Changes the Date
A first-trimester ultrasound (up to about 14 weeks) is the most accurate method to confirm or revise a due date. The technician measures the embryo from head to tailbone, a measurement called crown-rump length, and that size correlates tightly with gestational age. In the first trimester, this measurement is accurate to within about 5 to 7 days.
That might not sound dramatically better than counting from your last period, but it makes a real difference. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from the period-based calculation by more than five days. The American College of Obstetricians and Gynecologists considers any pregnancy without an ultrasound before 22 weeks to be “suboptimally dated.” In practice, your provider will compare the ultrasound estimate with your period-based estimate and use whichever is considered more reliable for your situation.
Accuracy drops as pregnancy progresses. By the second and third trimesters, babies grow at increasingly different rates, so size-based estimates become less precise. That’s why early ultrasound dating matters: it anchors the timeline before individual growth variation muddies the picture.
IVF and Assisted Reproduction Dating
If you conceived through IVF or another assisted reproductive technology, the calculation works differently because you know the exact date of fertilization or embryo transfer. There’s no guesswork about when ovulation happened, so the usual two-week buffer built into Naegele’s rule doesn’t apply. Your due date is calculated directly from the transfer date, and the ART-derived gestational age is considered the most accurate starting point. Clinics don’t differentiate between fresh and frozen embryo transfers when estimating the due date for blastocyst-stage embryos.
Factors That Shift How Long Pregnancy Lasts
Even a perfectly calculated due date assumes that all pregnancies last the same amount of time, which they don’t. Research has identified several biological factors that reliably influence gestation length. Maternal age, whether you’ve given birth before, and race are among the strongest predictors. First-time mothers tend to carry slightly longer than those who’ve given birth previously. Women younger than 19 or older than 34, on average, have shorter pregnancies than those in the 19-to-34 range.
These differences aren’t huge individually, but they stack. A 36-year-old having her second baby has a meaningfully different statistical profile than a 25-year-old having her first, yet Naegele’s rule gives both the same 280-day estimate. More refined statistical models that account for these variables can predict gestation length more precisely, though they haven’t replaced the standard formula in everyday clinical use.
How Likely You Are to Deliver on Your Due Date
The short answer: not very. About 70% of babies are born before their due date. Roughly 10% of deliveries are preterm, meaning before 37 weeks. The remaining 60% arrive in the weeks leading up to the due date, clustering most heavily in weeks 38 through 40. If you do reach your exact due date without delivering, there’s about a 60% chance you’ll give birth within the following week.
Thinking of your due date as the center of a window rather than a target helps set realistic expectations. Full-term birth spans from 37 weeks through 42 weeks, a range of more than a month. Your estimated due date sits roughly in the middle of that window. Most people will deliver somewhere within two weeks on either side of it.
Getting the Most Accurate Estimate
You can improve your due date prediction by layering a few pieces of information together rather than relying on any single method.
- Track your cycle before conception. Knowing your average cycle length lets you adjust the standard formula. If you tracked ovulation with test strips or basal body temperature, that data is even more useful.
- Get an early ultrasound. A scan between 8 and 14 weeks provides the tightest margin of error. If you have one, it will likely become the basis for your official due date.
- Factor in your history. If you’ve had previous pregnancies, how long they lasted is one of the better predictors of how long this one will last. Gestation length tends to be consistent within the same person across pregnancies.
- Expect a range, not a date. Even with the best data, the biological variability in human gestation means a single date will almost never be exactly right. Planning for a window of two to three weeks gives you a more realistic timeline for practical preparations.